[IMPROVE] assign_roS, [FEATURE] divgrad function
[IMPROVE] assign_roS, [FEATURE] divgrad function

only for cpu version

Signed-off-by: Anastasia Lyupa <866ee7be324b1e59601c43ba8575ecaf5812adb3@gmail.com>

alyupa/multiphase-flow-modeling,alyupa/multiphase-flow-modeling,alyupa/multiphase-flow-modeling,alyupa/multiphase-flow-modeling